1. Field of the Invention
This invention relates to air pollution control. More specifically, the invention is directed to a reactor and method for insuring oxidation of oxidizable materials in waste gas streams, and particularly by means of a waste gas purifier such as a catalytic oxidation unit. More particularly, the invention relates to a reactor and method whereby a catalyst bed of substantially uniform thickness is maintained by means of a combined leveling and packing holder.
2. Description of the Prior Art
Considerable effort has gone into removing harmful materials present in waste gas streams such as would be present in industrial plants such as those waste gases produced in the production of maleic anhydride. In developing waste gas purification units for such waste gas streams, one problem that arises is the low percent oxidation of the oxidizable components of the waste gas streams. This is particularly true in catalyst systems where it is necessary to make the catalyst beds relatively thin, such as from about 1 to 8 inches in depth, in order to prevent catalyst attrition and due to the high cost of the catalyst. Also, in some of these catalyst beds, such as those composed of oxides of manganese and copper, in order to prevent overheating of the catalyst and to minimize the energy needed to preheat, the waste gas is preheated only to the temperature needed to oxidize one of the contaminants, e.g. carbon monoxide (CO). The heat released by oxidation of the CO heats the gas to the ignition temperature of the other contaminants--hydrocarbons. Thus, the portion of the catalyst bed able to completely oxidize the hydrocarbons is small, which means that only a very thin layer of the catalyst bed is useful for conversion of hydrocarbons. If the catalyst bed thickness is not substantially uniform, the CO conversion will tend to decrease in thin areas and therefore provide less heating of the gas. The lower gas temperature in turn will diminish or perhaps prevent the oxidation of hydrocarbon pollutants.
This problem of non-uniform thickness of catalyst beds manifests itself in two ways. First, since the support screens for the catalyst bed are removable for ease of cleaning the screens and changing the catalyst, granules of the catalyst bed are lost over the edge of the catalyst support. Second, there is a problem of getting the catalyst bed level initially.
An example of the problem was demonstrated when in the construction of a catalytic oxidation unit it was discovered that, even with the close tolerances specified, gaps existed between the catalyst support screen, castable refractory sidewalls, and screen support gridwork. These gaps would have provided a path by which the small granules of catalyst could pass through thereby becoming lost from the catalyst bed rendering the bed ineffective. Also, because of distortions in the screens, grids and refractory, the desired spacing between the screen and the top of the refractory was unequal and outside the specified limits, thereby making it impossible to obtain the desired thickness of catalyst bed. Also, closer tolerances could not be specified because of necessary allowances for differential thermal expansion of the screen, vessel wall, grid and refractory.